For the production of liquid varnishes, paints, coating materials, emulsion paints, and printing inks containing metallic effect pigment, or for the coloring of plastics, platelet-shaped metallic effect pigments, usually in the form of dry powders or in the form of pastes, are incorporated into the formulation in question. These metallic effect pigment pastes usually comprise organic solvent and/or water as a pasting medium. With these two presentation forms of the metallic effect pigment, however, the process step of pigment incorporation has decisive disadvantages. Metallic effect pigment powders generate a considerable dust load at the production venue, resulting in increased workplace safety requirements. Metallic effect pigment pastes exhibit disadvantages in metering and, in the case of organic solvents as the pasting medium, generate a considerable odor nuisance at the production venue, and contribute to an increased VOC load in the environment. With both presentation forms, furthermore, there are disadvantages in transportation, owing to the considerable risk potential of a dust explosion and to the combustibility of organic solvent, respectively.
Even during relatively short storage times of several weeks to a few months, and particularly in the fine particle-size range, i.e., with metallic effect pigments having a relatively low pigment diameter, conventional metallic effect pigment pastes display a tendency toward aggregation, in other words toward the formation of intergrown primary particles which are positioned flatly against one another, which cannot be reversed in the course of incorporation into the end product in question. A short guaranteeable shelflife is the consequence. Moreover, homogeneous distribution of a metallic effect pigment powder in the application medium in question is often difficult, if not impossible, to achieve. In varnishes or plastics, for example, agglomerates of the pigment powder occur that cannot be deagglomerated under standard processing conditions.
In the case of processing in aqueous application media, it is always necessary to suspend metallic effect pigment powders or metallic effect pigment pastes in suitable cosolvents in order to ensure optimum pigment dispersion. Only the incorporation of this suspension into the aqueous coating formulation ensures homogeneous distribution of the pigments in the coating formulation and hence the development of the desired shade or metallic effect.
Conversely, and especially as part of the efforts to reduce the VOC loads (VOC: volatile organic compound), there is an urgent need to make further significant reductions in the fraction of organic solvents, even in waterborne coating materials.
There is therefore a demand for largely solvent-free presentation forms of metallic effect pigments for waterborne coating materials.
Many of the disadvantages identified can be circumvented or alleviated by means of a usually dry pigment preparation.
EP 1 517 934 A1 describes pigment preparations which in addition to pigment contain particular surface-active substances, which, however, are anionic or nonionic in nature or consist of mixtures of both. These preparations feature ready dispersibility in a wide variety of application media. Disadvantageously, however, it is not possible to provide dimensionally stable, compacted presentation forms of these pigment preparations. A further disadvantage of these pigment preparations lies in the high level of anionic and also, where used, nonionic additives, which may have an unwanted influence on the varnish or paint system into which the pigment preparation is incorporated.
Furthermore, with regard to the method for producing the pigment preparation, wet comminution in aqueous suspension is described. In the case of the use of platelet-shaped metal pigments, the mechanical comminution that occurs subjects the primary particle size of the pigment ready-produced per se—for example, the metal flake substrates provided with the desired coatings—to such severe alteration that the parameters or properties desired for the application, such as corrosion stability and optical quality, for example, are no longer fulfilled. For example, the wet comminution destroys any pigment coatings, and the reaction that occurs between metal pigment and water produces a sustained and adverse alteration in the character, more particularly the optical properties, of the metallic effect pigment. In none of the possible applications of such metallic effect pigment products is it possible any longer to achieve the desired metallic effect which would have been achievable if the uncomminuted starting metallic effect pigment powders or pastes had been used. The chemical stability of such preparations, containing coated metallic effect pigments, is no longer a given in aqueous applications, as a result of the destroyed coating.
EP 0 964 895 A1 refers to metallic effect pigment preparations which first and foremost are pastelike but which may also be processed further to form dry products. These pigment preparations comprise organic polymers in general form. The pigment content of the pigment preparation described in EP 0 964 895 A1, at only 40%-60% by weight, is—disadvantageously—relatively low, and this entails disadvantages for the user of the pigment product. As a result of such a low level of pigment in the product, the user has to accept a high fraction of extraneous substances in his or her formulation, and these substances may adversely influence the desired properties of his or her coating formulation.
DE 199 47 175 A1 describes a preparation with effect pigments which comprises as its resin component a styrene-modified maleate resin. The parent structure of the resin in this case is composed of maleic anhydride and styrene. Of styrene-containing binders, however, it is known that the weather stability of a coating deteriorates as the styrene content increases.
Furthermore, dry metal pigment products described in DE 199 47 175 A1 exhibit deficient pellet strength and disintegrate in service, with considerable dusting.
EP 1 104 447 A1 describes pigment preparations with polyalkylene glycols and also hydroxyl-rich binders. In the case of the dry products described, based more particularly on pearlescent pigments, the polyalkylene glycols, as for example polyethylene glycol 2000, do result in an improvement in strength, but impair the shelflife of the dry product such that, particularly when metallic effect pigments are used, there is formation after just a short time of pigment aggregates which can no longer be redispersed in the varnish. The loss of opacity through aggregation is high, and thus the shelflife is unsatisfactory. Furthermore, in paints and varnishes, polyethylene glycol has the disadvantageous effect of low weathering stability.
The dispersibility of these dry products in coating materials is difficult and requires high shearing forces and is therefore complicated and costly from the standpoint of process engineering. A disadvantage is that, with the high shearing forces that are necessary, there may be instances of damage to the pigments, more particularly to the pigment surface. Furthermore, applications of these polyalkylene glycol-containing products in coating materials are less brilliant than of coatings produced using corresponding powders or pastes which are free from polyalkylene glycol.
European patent EP 1 153 995 discloses a pigment preparation with effect pigments which as its resin component has a modified rosin with particular properties in terms of acid number, hydroxyl group content, and softening point. In the case of metallic effect pigments, however, these preparations do not exhibit high dimensional stability. The dispersing and opacity, i.e., hiding of a substrate per unit weight of pigment, in waterborne coating materials is likewise unsatisfactory. The softening range of the resins used, furthermore, of 50-120° C., is a disadvantage in relation to the operation of producing compacted pigment preparations. In this low temperature range, in the course of the drying of the pigment product, there is a risk of softening and blocking of the resin. Accordingly, the compacted pigment preparation is no longer free-flowing. Additionally, metallic effect pigments which have “sintered together” with softening of the resin are difficult to disperse, or cannot be dispersed at all, in a varnish system.
Particularly in the case of metal pigments, and especially for aqueous applications, the specifications cited in the prior art do not result in satisfactory solutions.
WO 85/00614 describes the production of pellets with metal pigments. Disadvantageously, however, no useful applications are disclosed therein for aqueous systems.
DE 196 16 287 A1 describes the production of pearlescent pigment preparations for printing inks. Here too, however, there are no possible solutions for aqueous coatings with metallic effect pigments.